[JS IR BE] Fix and refactor interop tests

js/js.translator/testData/box/rtti/prototypeUsedToFindInterface.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 // EXPECTED_REACHABLE_NODES: 1224
 interface A {
     fun foo(): String
@@ -10,13 +9,14 @@ class B : A {
 
 fun box(): String {
     val b = B::class.js
-    val c = js("""
+    val c = js("""(function() {
     function C() {
         b.call(this);
     };
     C.prototype = Object.create(b.prototype);
     C.prototype.constructor = C;
-    new C();
+    return new C();
+    })()
     """)
 
     if (c !is B) return "fail: c !is B"